In recent years, various copyright protection techniques for copyrighted digital audio materials have been established by SDMI (Secure Digital Music Initiative). According to the SDMI, copyrighted digital audio materials are converted into SDMI protected contents and are transmitted through the Internet. The SDMI protected contents are audio data that is protected against fraud and illegal playback by third parties, and the playback of the SDMI protected contents is managed by the audio data playback management system. Once copyrighted digital audio materials are converted into SDMI protected contents, only devices compliant with the SDMI can play back the SDMI protected contents. That is, devices that are not compliant with the SDMI cannot play back, let alone copy, the SDMI protected contents. Therefore, a special process called check-out needs to be performed to convert the SDMI protected contents into playable audio data. Check-out is performed by the stated management apparatus in the audio data playback management system. After obtaining an SDMI protected content via a network, the management apparatus performs check-out to convert the SDMI protected content into playable audio data, and records the audio data in a recording medium, such as a semiconductor memory card storing unique identification information. A playback apparatus plays back the audio data in the recording medium.
The number of times check-out can be performed (the number of permitted check-outs) is limited to a predetermined number, such as one, two, or three. Therefore, once check-out has been performed the number of permitted check-outs, the management apparatus prohibits further check-out and waits for check-in to be performed on the audio data in the recording medium. Check-in refers to a process of returning audio data, which has been set in a playable state, into a non-playable state. If check-in is performed on an SDMI protected content whose check-out is currently prohibited, the number of permitted check-outs for the SDMI protected content is incremented and check-out of the SDMI protected content becomes possible again.
To properly perform check-in, the management apparatus needs to judge whether audio data to be checked in from a recording medium is the same as that the management apparatus previously checked out to the recording medium. However, there is no conventional technique that uses an information system of managing check-out operations performed by devices. As a result, there may be cases where check-in is performed improperly on audio data in recording media. To solve this problem, techniques are recently proposed that manage check-out operations using identification information stored in recording media. The following is a description of how the correspondence between audio data is judged using the identification information in recording media. When check-out is performed to convert an SDMI protected content into playable audio data, the management apparatus assigns identification information (called a “track ID”) to the audio data. The audio data assigned the track ID is managed as a track in a recording medium. The management apparatus then reads unique identification information (called a “media ID”) from the recording medium and holds a pair of the track ID and the media ID as check-out history information.
If being instructed to check in the track recorded in the recording medium afterward, the management apparatus reads the media ID unique to the recording medium and the track ID assigned to the SDMI protected content from the recording medium and judges whether the pair of the read IDs matches the check-out history information. If the judgement result is affirmative, the management apparatus determines that the track to be checked in is audio data that the management apparatus previously checked out. In this case, the management apparatus sets the track in a non-playable state and increments the number of permitted check-outs for the SDMI protected content. If the judgement result is negative, check-in is not performed on the SDMI protected content.
With this technique, however, the management apparatus can properly perform check-in on a track recorded in a recording medium only if the correspondence between the track and an SDMI protected content is not changed. That is, if the track recorded in the recording medium has been edited and the correspondence between the track and the SDMI protected content is changed, the correspondence between the track and the SDMI protected content cannot be detected by referring the check-out history information described above. For instance, if the track in the recording medium is divided and a new track is generated, the new track is assigned a new track ID. In this case, even if the management apparatus attempts to check in the new track in the recording medium, it is judged that a pair of the track ID of the original track and the media ID of the recording medium matches the check-out history information and only the original track is set in a non-playable state. That is, it is judged that a pair of the new track ID and the media ID does not match the check-out history information, so that the new track assigned the new track ID is left in the recording medium as playable audio data. This makes it possible for an ill-intentioned user to illegally increment the number of permitted check-outs for the SDMI protected content. Therefore, there may be cases where the copyright to the SDMI protected content is infringed.
As described above, if a track checked out to a recording medium has been edited, the correspondence between tracks before and after the editing cannot be detected properly. As a result, an audio data playback management system conventionally needs to be produced according to one of (1) a specification where “check-in of tracks from recording media is allowed but the editing of the tracks is prohibited” and (2) a specification where “check-in of tracks from recording media is prohibited but the editing of the tracks is allowed”. This imposes inconvenience on users with considerable frequency.